As stated above, a printed circuit board of the kind described in the first paragraph is known, for example, from U.S. Pat. No. 4,528,064. It is true that said patent indicates in a general way the materials which comprise the support plate and the adhesive layer, but no guidance whatsoever is provided on what properties these materials, especially the materials of the adhesive layer, should have. The support plate and the adhesive layer of the known printed circuit board consist of materials with such properties that all sections, such as conductor tracks, solder pads and solder lands of the first conductor pattern lie at one and the same level in their entity. Additionally, after the pressing process for connecting the adhesive layer to the support plate, for example, these sections are perfectly plane and lie at a raised level relative to the support plate and project from the support plate at least for a substantial part. In practice there is a risk in the known printed circuit board that the sections of the first conductor pattern are pressed too deeply into the adhesive layer during the said pressing process and consequently the lateral edges of the conductor tracks, solder pads and solder lands, which are comparatively sharp, have pierced through the adhesive layer, given the thickness of the adhesive layer. As a result at least portions of the first conductor pattern have penetrated through the adhesive layer. As a result, a disadvantageous short-circuit between the penetrating sections of the first conductor pattern and sections of the second conductor patterns supported by the adhesive layer may arise. To prevent such a penetration of sections of the first conductor pattern through the adhesive layer, a suitably thick adhesive layer could be provided, which, however, is also disadvantageous, especially for obtaining perfect electrically conducting material connections, especially soldered joints, between the connecting sections of the two conductor patterns, and for obtaining the highest possible mechanical strength of the adhesive layer and the lowest possible material cost.
The fact that the sections such as conductor tracks, solder pads and solder lands of the first conductor pattern in their entirety lie at a raised level relative to the support plate may also have the result that the adhesive layer either does not form or forms only an unsatisfactory adhesive connection not only with the conductor tracks, solder pads and solder lands of the first conductor pattern, but also with the regions of the support plate situated adjacent the conductor tracks, solder pads and solder lands. This is disadvantageous especially in the case of a densely arranged first conductor pattern, because then unsatisfactory mutual adhesion between the support plate and adhesive layer may arise over comparatively large surface areas. This may result in gas inclusions which upon subsequent heating, for example during a soldering process for manufacturing the electrically conducting material connections between the connecting sections of the two conductor patterns to be electrically interconnected, expand as a result of the high temperatures occurring during such process. Such expansion of gas inclusions disadvantageously leads to an undesirable partial detachment of the adhesive layer from the support plate, and consequently to a removal of the second conductor pattern supported by the adhesive layer from the first conductor pattern so that in these regions no soldered joints between the connecting sections of the first conductor pattern and the second conductor pattern to be interconnected is achieved.
Moreover, since all sections of the first conductor pattern are perfectly plane in their entirety, in the known printed circuit board, furthermore, the first conductor pattern and the second conductor pattern lie in two comparatively strongly differentiated levels. This may have the result that in the manufacture of a soldered joint as an electrically conducting material connection between a connecting section of the first conductor pattern and a connecting section of the second conductor pattern through an opening in the adhesive layer, (which may take place, for example, in a wave soldering process), the solder does indeed achieve a soldered joint with the exposed connecting section of the second conductor pattern, but does not achieve a soldered joint with the connecting section of the first conductor pattern lying at a deeper. This occurs because of the formation of a gas bubble over the connecting section of the first conductor pattern lying at a deeper level, so that an electrically conducting connection through the soldered joint may not be obtained. The soldering failure then leads to a defective printed circuit board, which must either be discarded or repaired in a separate subsequent soldering process, both being obviously disadvantageous and undesirable. Such a soldering failure is especially disadvantageous when this soldered connection completely covers the connecting section of the first conductor pattern, because in that case the soldering failure with the connecting section of the first conductor pattern cannot be visually observed, so that it cannot be discovered even in an optical product check.